Relativity tested on a shoestring budget

A NASA mission costing $600 million, which aimed to be the first to measure an esoteric prediction of Einstein's theory of general relativity, has been pipped to the post by a pair of scientists on a shoestring budget.

Einstein's famous theory says that a massive rotating body should drag space-time around with it. The phenomenon - known as the Lense-Thirring effect - would cause the axis of spin of a gyroscope orbiting the Earth to go out of alignment by an angle of about 42 milli-arcseconds per year, equivalent to the width of a human hair seen from a kilometre away.

To test for this effect, NASA and Stanford University physicists conceived Gravity Probe B - which carries high-precision gyroscopes - in the early 1960s but technical and financial problems delayed the launch until April this year.

Meanwhile, Ignazio Ciufolini, a physicist at the University of Lecce in Italy, and Erricos Pavlis from the Joint Center for Earth Systems Technology in Baltimore have been quietly measuring the Lense-Thirring effect using data from two NASA satellites that were sent up on an entirely different mission.

The LAGEOS satellites, one launched in 1976 and the other in 1992, are small shiny balls that reflect laser range-finding signals back to Earth. The reflections, which reveal small changes in the orbits of the satellites, are used to map variations in Earth's gravitational field.

Complicating factors

Their orbits should also be altered by the Lense-Thirring effect and Ciufolini's idea was to find evidence for it in the range-finding data. However, the orbits of the satellites can also be affected by the Earth's magnetic field, the thermal radiation emitted by the satellites as they are heated by the sun and small variations in Earth's gravity. "Any of these can obscure the Lense-Thirring effect," admits Pavlis.

Nevertheless, he and Ciufolini say they have allowed for these and have seen the effect after analysing 11 years of data from the LAGEOS satellites. And because the work was mainly analysing existing data, it cost essentially nothing, says Pavlis.

Francis Everrit, leader of the Gravity Probe B mission at Stanford University, declined to comment on the work but pointed out that a similar claim by the team in 1998 was severely criticised. At the time many observers pointed out that the maps of Earth's gravity field were too imprecise to allow the effect to be seen. But Pavlis says their latest work is based on a new gravity map released last year. "We've removed the ammunition they used against us," he says.

Others agree. David Rubincam, a geophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, says the new analysis is a significant improvement on previous work. "They have a better gravity model and the analysis looked reasonable to me."

Journal reference: Nature (vol 431, p 958)

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.